The present invention has particular application in low pressure environments where accuracy of the measurements are important. In use of a pressure gauge, sudden pressure changes can also typically accompanied by a temperature change in a relatively short period of time. It is also important for the accuracy of the pressure sensor to not change its calibration (i.e. repeatability) over a period of time in response to pressure or temperature effects. Thus, there is a need for a pressure gauge for pressure measurements which is also insensitive to sudden changes in temperature or repeated effects of pressure over a period of time. The present invention relates to pressure transducers which can accurately measure pressure changes under transient temperature conditions independently of the temperature changes.
Under the combined effect of temperature and pressure conditions, the typical pressure sensor structure is subjected to stress by the applied pressure and subjected to temperature which together cause creep (permanent distortion) in the materials of the sensor structure. Creep in materials tends to be exponentially related to both temperature and stress levels. The effect of creep or permanent deformation in materials is to alter the calibration or measurement characteristics (repeatability) of a sensor and cause the sensor to obtain inaccurate measurements relative to its calibration standard over a period of time.
Heretofore, capacitance type transducers have been utilized for pressure measurements where pressure varies an electrical capacitance as a function of pressure. The pressure is translated to a capacitance measurement by a pressure diaphragm moving parallel arranged capacitance plates toward and away from one another. An example of the kind of device is shown in U.S. Pat. No. 4,322,775.
I have also coupled a bourdon tube to a capacitance type of sensor such as disclosed in U.S. Pat. No. 4,873,870 in which the pressure in the bourdon tube generates a directional linear force to displace quartz supported parallel arranged capacitor elements toward and away from one another. While this device is satisfactory for a number of applications, it is a difficult unit to manufacture.
Bourdon Tubes (as contrasted to a bellows) are also good for measurement of high pressures but at lower pressures of 500 psi or less the physical size of the bourdon tube required to obtain a response increases dramatically. Bellows, on the other hand are not suitable for high pressure measurements but are practical for sensing pressures to about 500 psi.
In the present invention, I have developed a capacitor device which has a unique relationship of capacitors that can be constructed from metals to respond to low force inputs and be relatively insensitive to temperature changes. The capacitor device is particularly adaptable to measurement of low pressure with a high degree of accuracy and repeatability over a period of time.